Pete Millett NuHybrid Headphone Amp

Do It Yourself (DIY & Mods)
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Type: Hybrid Tube Headphone Amp & Preamp
Budget: around $100 using stock parts, $150 if using TKD pot
PSU: External, 24V, 1A (switching)

Datasheets

Links

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I had posted on other threads regarding this amp, but as @Luckbad pointed to me it surely deserves its own thread, moreso after @Torq intention of building it :smile:

Really easy, unexpensive and safe project, powerful amp with low, euphonic distortion and sweet tube Single Ended sound w/o cloying nor breaking the bank, or having to hunt unobtanium NOS tubes to get it ā€œrightā€.

Itā€™s unexpensive because thanks to Pete Millett DIY spirit you basically get the PCB for free, the NuTube is worth $50 alone.

Been following Pete Millett designs since early 2000ā€¦ since then I have a crush with his designs, have to try them, budget allowing, of course.

When saw this new design, I had to try it!

Uses the Korg developed, Vacuum Fluorescent Display (opposed to classic Thermoionic Effect used by traditional tubes) technology based NuTube 6P1

nutube_small

NuTube provides the gain (6x) stage (A1), relaying the heavy lifting to OPA551 opamps at the output.

The NuHybridā€¦ was assembled in a couple hours, was my Xmas project. I wholeheartedly advice this project for anyone wanting to step into DIY tube sound safely and unexpensively.

I just upgraded the BOM to WIMA MKP caps (+$2, fit perfect) and the TKD pot, as the weak link on most amps is the potentiometer. Only mod done was grounding the TKD metal case to the PCB ground.

Use a typical POE brick (24V 2A) from a mikrotik router to power it. Tried a linear PSU, saw zero enhancement, so back to the switched one.

Its sound (HD6XX)ā€¦ itā€™s incredibly transparent, while adding just the proper amount of euphony, keeping pace and decay without losing slamā€¦ I was more blasted after hearing the NuHybrid than after my last OTL White Cathode Follower monster iteration using almost 6A in heaters.

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Some tips:

Caps
Only mod from stock BOM Iā€™d change straight away is upgrading from 505-MKS2.1/63/10 8 to 505-MKP2D031001F00KF

You can tweak with bigger bypass caps afterwards by soldering on the underside of the PCB, most significant positions being C4,C7 and C20,C21.

Volume Pot
Bear in mind replacing the pot once soldered to the PCB is a no-noā€¦ Iā€™d go straight to the TKD if youā€™re going to case it as ā€œstockā€. (stock BOM pot control knob wonā€™t fit in this case, youā€™ll need a 6mm round shaft one)

Stepped attenuators (or pots with detents) are a no-no with this amp, any sharp vibrations on the PCB will make the NuTube ring.

Microphonics/Vibration Isolation
Though some seem to have had issues with the NuTube microphonics, it hasnā€™t been my case; I just hear a distant zingggg for about 1 second at power on (induced by the power switch vibration). Completely normal and absolutely acceptable.

If going with alternate casing with power switch and stepped att/pot, connectors etc decoupled from the PCB, this could be alleviated by using rubber or silicon grommets sandwiching the PCB at its fixing holes.

Couldnā€™t harm, specially if you plan to use it as preamp, where vibrations can be induced by the speakers.

Grounding mod
I would also ground the pot case to a PCB GND pad, easily done with a e.g. cap leg trim caught between the pot shaft shim/nut, soldered to a free GND pad (as PCB has two different pot footprints you always have one free: easy peasy).

PSU
24V PSUs are really common for PoE and Telco routers (mikrotik); if you happen to have any friend or relatives working in telco, (W)ISP or network center, ask firstā€¦ as when using PoE switches/panels you end up with a pile of unused and very fine 24V/2,5A adapters.

eBay should have tons of them too, just search 24HPOW. (They go for around $18 new)

I liked this amp so much I plan to build several more to give as gifts to relatives!

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Nice! Have you made an enclosure to it? Or is there a kit with enclosure available? Last time I tried to drill out my own custom enclosure, I totally messed it up. Haha.

This is my DIY Millet Starving Student kit I built last summer: https://www.antdroid.net/2018/08/pete-millet-starving-student-hybrid.html

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No, the Project Bom includes a small plastic box, whose deeper part serves to screw the PCB, otherwise is an exposed design.

As there are no high voltages, it is completely safe.

Built an Starving back in the days too (a couple, gifted to relatives)! :smile:

This project is really solder by numbers, and SQ is on a very different level (also Vs Millett Hybrid Max). OPA551 in this implementation is no slouch, if I recall correctly theyā€™re capable of delivering 500mA each, and more importantly, according to Pete:

Using the OPA551 removes a little of the high-order distortion products that were caused by the BUF634 in the original hybrid.

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Last time I tried to drill out my own custom enclosure, I totally messed it up. Haha.

Hey that starving looks slick! Wouldnā€™t say you messed it!

Case working isā€¦ the last (and sometimes never done) part :laughing: Have like 3 - 5 projects needed half-finished still due to the casingā€¦

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Thanks for your extensive, and comprehensive review. Experience, hands on, and trial and error are the best ways to provide a through review.
Sad to say in all the years in this hobby, I have failed to muster the courage to do diy projects. Some day maybe?

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This would be an ideal first project. :smile:

Iā€™m sure the sense of achievement will pay for itself.

Back in the days, ā€œaudio nutsā€ were used to hack/DIY their own ā€œtunedā€ rigsā€¦ Iā€™d say half of the fun/reward was in building them yourself!

Just follow the assembly manual to the letter, double and triple check, youā€™ll succeed.

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That was a custom machined box from Beezar Audio. :slight_smile:

I have since changed the volume knob to a larger machined silver aluminum knob and it looks great now.

As for the nuhybrid, I will put it on the list for a fun build.

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My boards and Nu-Tubes arrived from Pete (hadnā€™t even realized they had shipped). The rest of the parts should be in tomorrow. Not sure when Iā€™ll get to put one together, but it looks like a pretty quick build ā€¦ so maybe right after I get my AFC/AFO review posted.

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Sorry about the cell-phone pictures ā€¦ the boards are identical in color and finish, the difference is just down to uneven lighting.

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I have these in my cart, I just need to figure out where to get all the appropriate parts and the better POT that was mentioned. This weekend turned busy quick and I havenā€™t been able to dedicate time to it.

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@pukkita has links in his post thatā€™ll let you buy all the parts you need, except the board and tube. Iā€™ll repeat it here, though:

Pete Millet NuTube Hybrid - BoM (Mouser)

Click that, hit the ā€œOrder Projectā€ button, and itā€™ll add everything you need to your cart. Iā€™d recommend setting up your Mouser account first, if you donā€™t have one already.

You can get the upgrade TDK pot via PartsConnexion here. If you order one of those, you can remove the stock Alps pot from the above BoM (or your cart).

If you want the upgraded WIMA caps, then remove the set of 8 such caps from the original BoM and replace them 8 of these.

Iā€™m building one off the original BoM and one with the upgrades.

Also, note that there are some pretty small pads on the PCB. Youā€™ll want a much finer tip on your iron than the standard one that comes with a Hakko FX-888D or the various Chinese clones. I use a micro-pencil for such things so donā€™t know the specific Hakko 888D or 3rd party tip part numbers for your station.

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@Torq

You arenā€™t making this easy for me to avoid by breaking my self imposed barriers of entry lol.

Iā€™ll order everything tonight thenā€¦ thank you for the heads up about the mouser account, this person doesnā€™t also go by the name Dr. Baxter Stockman perchance?

Wellā€¦ā€œonce more unto the breach, dear friends, once moreā€

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Forgot to comment on that, although no SMD parts to be soldered here, small wattage (30/50W or temp controlled) iron required, fine chisel tip definitely a plus.

For The Hakko: T18D16P

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https://www.amazon.com/Hakko-T18D16P-Fx-888-Station-1-6mm/dp/B004OR9BV4

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I sourced lots of parts for the Starving, grubDAC, and Millet Hybrid Max from Tomb at Beezar too!

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Whatā€™s the estimated life span of the nutubes?

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According to Korg:

Made in Japan. 30,000 hours of continuous life expectancy

Just learnt there are a couple Vox instrument amps using them: VOX MVX150H and VOX MVX150C1.

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Well all parts ordered and upgrade parts, plus new soldering tips. Apparently my self imposed lazy barriers werenā€™t all that difficult to break downā€¦ mix in a cool project, plus decent return on investment (for me the fun of building a thing, that I find cool, plus potentially good fun portable amp).

Hopefully I will get everything this week or the next and Iā€™ll start putting it togetherā€¦plus this might get me to finally put my Speedball together, and reflow all my joints on my Bottlehead Crack.

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The nice thing about PCB-based builds is that, in general, they go together way faster and more easily than point-to-point wiring builds. Other than the much smaller connection points, the physical build is easier even than the Crack.

When dealing with any kind of solid-state semiconductor I do make sure Iā€™m not a) risking static discharge issues (so a properly setup mat and wrist-strap are useful ā€¦ theyā€™re not expensive) and b) Iā€™m watching my dwell-times on soldering them.

For the NuTube Hybrid, the op-amps are socketed, so thereā€™s no thermal risk there. Diodes are generally pretty hardy, but transistors are typically less so.

And, as ever, use leaded flux-cored solder ā€¦ the lead-free stuff will just make your life a misery, and heat the joint, not the solder!

On tips ā€¦ Iā€™ll be using a 0.5mm tip for most of the connections, and then something a little larger for the RCA sockets, headphone jack, DC socket and pot.

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Definitely eutectic solder is one of the best DIYer weaponsā€¦ thankfully I bought a half kilo roll back when Pb wasnā€™t banned, stocked for a lifetime!

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Been listening to the NuHybrid with a new XMOS208/AKM4490 DAC I just got.

Even with the tubey ā€œgoodnessā€ is it clear NuHybrid remains a fairly transparent amp, that scales with the source. Good!

Tried the Chi-Fi AKM4490 DAC first with USB power, and then with a standalone supply.

A track I like specially for DAC testing: Kate Bush ā€œKing Of The Mountainā€ from

Differences were instantly spotted through the NuHybrid: much more coherent soundstage and consistently layered depth on it with standalone supply.

Amongst DarkVoice, NuHybrid and P20, the NuHybrid sounds really, really, really close to the OTL Darkvoice, possibly a tad clearer, with less aparent mid-veil, which I assume is coming from the DarkVoice stock output caps.

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